Selenium is a trace metal found in U.S. coals. It has been estimated that the concentration of selenium in U.S. coals has an overall average value of approximately 4 parts-per-million (ppm) by weight. The range of concentrations throughout coals found in this country is roughly 0.1 to 10 ppm by weight.
At the average concentration of 4 ppm by weight in the coal, combustion will produce a concentration of the oxide, SeO.sub.2, of approximately 0.12 ppm by volume in the flue gas. Alternatively, if the selenium is found as a component of the ash, the concentration in the solid will be about 50 ppm by weight.
Evidence suggests that little of the selenium will be discharged from the furnace in bottom ash and virtually all will be found in some form in the combustion gas, either as a vapor or as a solid component of the fly ash. The relative proportions of selenium in the gas state and in the fly ash will depend upon the temperature. As a general rule, the gaseous state will be predominant at high temperatures, and the solid state will prevail at low temperatures. Thus, the emission of selenium from a furnace operating with a coal having the typical selenium concentration will be something less than 0.12 ppm by volume of vapor and up to 50 ppm by weight in the fly ash.
The literature reveals different viewpoints on the chemical state of selenium in flue gas in the vapor state. Some authors hold that the vapor is the free element, occurring perhaps most abundantly at 150.degree. C. as Se.sub.6. Others argue that the predominant vapor form is selenium dioxide (SeO.sub.2), where the element exists in the IV oxidation state. The invention claimed here pertains to the control of that part of the vapor that occurs as the oxide.
The Clean Air Act of 1990 lists 189 air toxics that may be subject to control under new EPA regulations. Selenium is one of the 189 classes of pollutants. Presently, selenium emissions from power plants are not regulated; however, the U.S. Environmental Protection Agency is presently reviewing emissions of a broad range of potentially toxic materials, and regulations governing selenium as well as other metallic elements are a possibility in the near future.
Existing devices for particulate control include electrostatic precipitators (ESPs), baghouses, and scrubbers. These devices do not remove gaseous selenium by design. Whatever removal of selenium that occurs in these devices is incidental to the primary operation, which is the removal of fly ash or sulfur dioxide (SO.sub.2). Whatever removal occurs is more likely to involve the elemental state than the oxide of selenium, since the element is less volatile than the oxide. Scrubbers, however, may be more effective for capturing the oxide than ESPs or baghouses since SeO.sub.2 may dissolve along with SO.sub.2 in the scrubber fluid. A measure of the impact of the volatility of selenium on the total emissions of selenium is given by the result of one investigation of trace-metal collection in an ESP; the results of this investigation showed that 93% of the selenium passing the ESP and being emitted to the atmosphere was in the vapor state, largely as the oxide.